Planar raised cathode alpha-numeric gas discharge indicator

ABSTRACT

A gas discharge indicator comprising a cathode-supporting substrate and a parallel transparent anode spaced from and hermetically sealed to said substrate to contain an ionizable gas therebetween. Individual cathode segments having greater width than thickness are mounted on the tops of feed-through pins which penetrate the substrate in hermetically sealed relationship and extend beyond the opposite surfaces of the substrate whereby the individual cathode segments are spaced relative to the substrate as well as relative to the anode. The cathode-substrate spacing is such that the cathode segments glow only on the anode or viewable side of the cathode, thereby reducing the sputtering rate, and increasing the luminous efficiency. Additionally, a cup-shaped moat is formed in the cathode side of the substrate about each pin. The raised cathode and moat structure collectively and individually minimize the effects of sputtering and extend the operating life of the indicator.

United States Patent Armstrong et al.

1 July 4, 1972 [54] PLANAR RAISED CATHODE ALPHA- NUMERIC GAS DISCHARGE INDICATOR [72] Inventors: James B. Armstrong; Dan J. Schott; Le-

land C. Warne, all of Phoenix, Ariz.

[63] Continuation-in-part of Ser. No. 742,662, July 5,

1968, abandoned.

3,302,052 l/l967 Schwab ..3l3/l09.5

FOREIGN PATENTS OR APPLICATIONS 908,488 10/1962 Great Britain 313/210 Primary ExaminerRoy Lake Assistant ExaminerPalmer C. Demeo Attorney-S. C. Yeaton 57 ABSTRACT A gas discharge indicator comprising a cathode-supporting substrate and a parallel transparent anode spaced from and hermetically sealed to said substrate to contain an ionizable gas therebetween. Individual cathode segments having greater width than thickness are mounted on the tops of feed-through pins which penetrate the substrate in hermetically sealed relationship and extend beyond the opposite surfaces of the substrate whereby the individual cathode segments are spaced relative to the substrate as well as relative to the anode. The cathode-substrate spacing is such that the cathode segments glow only on the anode or viewable side of the cathode, thereby reducing the sputtering rate, and increasing the luminous efficiency. Additionally, a cup-shaped moat is formed in the cathode side of the substrate about each pin. The raised cathode and moat structure collectively and individually minimize the effects of sputtering and extend the operating life of the indicator.

4 Claims, 5 Drawing Figures Maloney et a1. 313/1095 PATENTEIJJUL 41972 3, 675,06 6

sum 10F 2 INVENTORS JAMES B. ARMSTRONG DA/V J. SCHOTT LELAND 6. WAR/VE ATTORNEY P'A'TENTEDJUL 41972 F lG.3b.

SHEET 2 BF 2 FZZZZZZ I I 35 F|G.3c.

INVENTORS JAMES B. ARMSTRONG DAN J. SCHOTT LELAND C. WAR/V5 AT OR/VEY PLANAR RAISED CATIIODE ALPHA-NUMERIC GAS DISCHARGE INDICATOR CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. patent application, Ser. No. 742,662, filed July 5, 1968, entitled Planar Raised Cathode Alpha-numeric Gas Discharge Indicator, now abandoned and replaced by continuing application Ser. No. l58,536 assigned to the instant assignee. This application is related to raised cathode gas discharge indicators disclosed in co-pending patent application entitled Planar Gas Discharge Indicator, Ser. No. 5,435, filed Jan. 23, 1970, and Method for Manufacturing Planar Raised Cathode Gas Tube, Ser. No. 5,436, filed .Ian. 23, 1970 and assigned to the instant assignee.

BACKGROUND OF THE INVENTION The invention relates to planar alpha-numeric indicators of the gas discharge type wherein the cathode substrate forms one member of a hermetically sealed package containing an ionizable gas. Cathode glows are viewed through a transparent cover member overlying and hermetically sealed to the cathode substrate. Prior devices of the aforementioned type suffered from want of adequate operational life. Cathode sputtered material was chiefly responsible for the premature failures, the sputtered material forming low resistance paths between adjacent cathode segments. Attempts have been made to lengthen operational life by the mixture of a heavy molecular weight gas such as mercury with the ionizable gas. While this technique resulted in extended life in certain gas discharge tube designs, it alone did not yield satisfactory operation in planar type indicators wherein the cathode segments are deposited, bonded, or otherwise rest upon a common substrate in closely spaced relationship.

SUMMARY OF THE INVENTION In accordance with the present invention, premature failure due to sputtering in planar alpha-numeric gas discharge tube indicators is avoided by mounting each cathode segment on top of a respective feed-through pin passing through a common cathode substrate whereby each segment is raised above the substrate. Thus, each cathode segment is spaced from both the anode and the substrate, the relative spacing of the cathode from the substrate being such that with a predetermined combination of gas pressure, and current density, the cathode segments glow only on the anode or viewing side thereof. For example, with a gas pressure of 50 mm I-Ig-neon, a 0.030 inch anode-cathode space and a 0.005 inch cathodesubstrate space will produce such anode-side glow only. To further reduce the effects of sputtering a cup-shaped moat is formed on one side of the substrate around the base of each cathode support pin. An additional advantage of the cathode to substrate spacing is that the power required to produce a given lumenous glow is reduced by approximately 50 percent compared to devices in which the glow surrounds or nearly surrounds the cathodes.

The cathode segment-substrate assembly is fabricated by providing a planar insulau'ng substrate having a plurality of feed-through pins, one end of the pins lying flush with the surface of one side of the substrate with the other end of the pins extending beyond the opposite surface of the substrate. The side of the substrate having the flush pin terminations is then etched away so as to expose the desired length of each pin above the etched substrate surface. During etching, a cupshaped indentation or moat is formed in the substrate around the base of each pin. The moats formed during the substrate etching process results from the use of a substrate-pin combination which produces compressively strained seals; these strained (glass) seal areas etching more rapidly than the unstrained portions of the substrate. To the tops of each of the feed-through pins a cathode segment is secured, as, for example, by welding. Each cathode comprises an elongated usually rectangular (ribbon shaped) metallic element having a substantial lateral width to thickness ratio. These segments may be arranged in any conventional alpha-numeric configuration as shown in FIG. 1.

It has been determined that the raised cathode segments having greater width than thickness substantially reduces the detrimental effects of cathode sputtering and significantly increases operational life of the indicator tube. The sputter products do not tend to coat the steep walls of the cup-shaped moat and therefore there is provided an annular surface around the pins that is substantially free of sputtered products and hence forms an insulating barrier between the sputtered and hence slightly conductive substrate surface and the pin. In other words, the cathode segments tend to remain electrically free of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a simplified exploded view in perspective of a typical embodiment of the invention.

FIG. 2 illustrates a typical cross section of a raised cathode gas discharge tube.

FIGS. 3a, 3b, and 3c illustrate the relationship of cathode to substrate spacing for limiting the extent of glow.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, glass substrate 1 is apertured to receive a number of electrical contacting pins 2 which are hermetically sealed to substrate 1. The ends of pins 2 on the viewed (near) side of substrate I extend above the substrate surface a predetermined distance; in the specific embodiment illustrated, a distance of about 0.005 inches. The pins extend below the back surface of substrate 1 a convenient distance suitable to being received by a mating pin receptacle (not shown) for the application of operating potentials. Individual cathode segments 3 are mounted on the ends of respective pins 2 to form a plurality of conventional alpha-numeric configurations. In addition to the alpha-numeric cathode segments, cathode segment 4 is provided to represent a decimal point; cathode segments 5 and 6 are provided to represent the letter L and cathode segments 7, 8, and 9 are provided to represent the letter R. The disclosed segment pattern is merely illustrative of one aircraft instrument embodiment of the invention. All cathode segments lie in approximately the same plane at substantially the same distance from substrate 1 on the ends of their respective pins 2. A cathode segment thickness of 0.005 inches has been found to provide satisfactory results but this thickness may be reduced depending on the manufacturing techniques and rigidity requirements.

In the preferred embodiment indicated in FIG. 1, the cathode segments 3, 4, 5, 6, 7, 8, and 9 are shown to have a generally greater length than width and to have a width to thickness ratio that may vary from more than 1 to I to 10 to l where the width is measured parallel to the plane of the substrate and the thickness normal thereto. This configuration presents several advantages. First, it provides a cathode segment size satisfactory for viewing purposes. Since the size of the cathode glow can be made to resemble the cathode segment configuration, the resulting viewed glow can be arbitrarily configured. Second, the segment tends to act as a shield to inhibit deposition of sputter products on the substrate about the cathode segment pin. If a portion of the moat is hidden from a line of sight from the edge of a cathode segment, the probability of sputter deposition on the hidden portion of the moat is remote. Third, the greater width than thickness of the cathode segment provides benefits in terms of manufacturing ease in orienting the cathode segments and attaching them to their respective pins. Fourth, the luminous effrciency (power required for a given brightness) is increased on the order of a factor of 2.

Substrate l is provided with an aperture 11 to which gas fill tube 12 is hermetically sealed on the side of the substrate opposite the cathode segments. A mercury ampule 13 is inserted inside tube 12. Finally, substrate 1 is equipped with a pair of hermetically sealed feed-through pins 14 and 15 for establishing electrical contact to the anode member of the gas discharge lamp. It is understood that other segment configurations may be used provided that the cathode segment to substrate spacing taught herein is followed.

Anode member 16 of the gas discharge lamp comprises a glass substrate on which is deposited a thin film 17 of a transparent conductive material completely covering the surface facing cathode substrate 1. Cathode substrate 1 is spaced from anode member 16 by glass spacer plate 18 having a main aperture 19 defining a volume of ionizable gas for producing cathode glows when the indicator of FIG. 1 is completely assembled and energized. Spring contacts 22 and 23 pass through a pair of smaller apertures and 21 to establish electrical contact from pins 14 and 15 to the conducting film surface of anode 16 in the assembled unit. In accordance with the teachings of the present invention, the thickness of the spacer plate 18 is determined by the anode-cathode, cathode-substrate space required to cause the cathode segments to glow, taking into account, of course, the thickness of the cathode elementszthemselves. In the illustrated embodiment the spacer thickness is 0.040 inches. Alternatively, the separate anode and spacer elements may be combined into a single molded unit, the side walls of which are ground to the dimension required to result in the above-discussed element spacing. In a further alternative construction, the transparent anode may be a separate fine wire mesh of, say stainless steel, welded to pins upstanding from the substrate such as pins 14 and 15, to a distance required to produce the described cathode to substrate spacing.

The indicator of FIG. 1 is assembled by bringing the elements 1, l6, and 18 into aligned contact with each other and then hermetically sealing the entire edge surfaces of the aligned units. Conventional techniques are applied for purging the atmosphere within the sealed unit and then filling and sealing the unit with an appropriate ionizable gas and an amount of mercury in ampule 13. The mercury is released into the sealed gas atmosphere at a suitable time during fabrication as is well known in the art. After release, the mercury provides an amount of heavy molecular weight gas which is believed to inhibit cathode sputtering by absorbing a significant amount of the kinetic energyof the ionized gas particles before they bombard the cathode segments. 1

Each of the cathode segments 3-9 is welded or otherwise secured to the top surface of a respective pin which protrudes from the surface of substrate 1 by an amount determined to provide the above spacing requirements; in the illustrated case, on the order of about 0.005 inches as previously mentioned. In accordance with the present invention, the tops of the pins to which the cathode segments are attached are made to lie along substantially the same plane parallel to the surface of substrate 1 by means of an etching process. In the process, the apertures in substrate 1 are filled with feed-through pins so that one end of the pins lies flush with the viewed surface of the substrate. The pins are then hermetically sealed to the substrate as by a glass-to-metal seal which produces compressively strained areas about the pin circumferences. The viewed surface of the substrate is then chemically etched to the required 0.005 in depth relative to the original pin surface plane whereby to expose the desired length of each pin above the etched surface.

The etching procedures simultaneously produces a cupshaped indention or moat about the base of each pin as the result of the fact that the etchant' attacks the stressed glass-to metal seal region around each pin at a faster rate than it attacks the glass substrate not in the vicinity of the glass-metal seal; i.e., the unstressed region of the substrate material. A solution of distilled water and hydrofluoric acid is suitable as the etchant.

Although it is believed that the more rapid etching of the glass is due to the strain about the pins, it is not conclusive. Some evidence indicates that there may be a metal ozide/glass interface layer between the glass and the pin which is more rapidly afiected during the etching process than the pure glass. Another possibility may be that a glass'of a different composition may be formed about thepins during the sealing of the pins which is less resistant to etching than the surrounding glass.

It has been detemiined that the rate with which cathodesputtered material deposits over the surface of the substrate 1 to an amount sufficient to establish low resistance paths between cathode segments is substantially reduced in the presence of the moats as compared to the rate obtained in the absence of the moats. It is believed that the reduced rate of low resistance path build-up is attributable to the increased surface area exposed between adjacent pins by virtue of the moats about the base of each pin. The sputtered products do not tend to coat the relatively steep walls of the moat thereby providing an annular insulative ring about the pins. Thus, for a given sputtering rate, the length of time required to deposit an objectionable conductive layer over the extended surface is substantially increased due to the moats and the previously discussed cathode segment configuration. The result is that the operational life of the gas discharge tube indicator is significantly lengthened.

While in the present embodiment the cup-shaped moats are formed as a result of the type of glass-to-metal seal used, which results in stressed areas about the pin, which in turn etch at a greater rate than the unstressed areas, it will be understood that many other techniques may be employed to form the moats without departing from the teachings of the present invention, for example, these techniques may be purely mechanical, purely chemical, or combinations of both techniques.

The theory of operation of the previously describe apparatus will now be described with reference to FIGS. 2, 3a, 3b, and 30. FIG. 2 illustrates a cross section of a raised cathode gas discharge tube constructed in accordance with the inven tion having a transparent glass cover 24, anode 25, anode electrical contact means 26, spacer 27, substrate 38, cathode segment 29, cathode support and electrical contact means such as pin 30, and a moat 31 disposed in the substrate and surrounding the pin 30. The invention relies exclusively on the cathode glow 32 (negative glow or plasma) for display presentation as shown in FIG. 2. The cathode glow 32 is generated in the form of a luminous conformal cloud about the cathode segment 29, and assumes the general outline of the cathode segment 29. The thickness of the cathode glow 32 in the cathode to anode direction is dependent on the combination of gas composition, pressure, electrode material, and current density. The existence of a cathode glow 32 between the anode 25 and cathode segment 29 causes the insulating substrate surface 33 of the substrate 28. in proximity to the cathode segment 29 to acquire a static charge. This charge establishes a field which repels the cathode glow 32. The intensity of the repelling field affecting the cathode glow 32 is a function of the cathode segment 29 to substrate 28 spacing such that the smaller the spacing the greater the intensity. The resultant effect of lowering or raising the cathode segment 29 is that of respectively inhibiting a greater or lesser portion of the cathode glow 32 from encircling the cathode segment 29. Thereby, the extent of the cathode glow 32 beyond the edge of the cathode segment 29 is a function of the cathode segment 32 to substrate 28 spacing. In the preferred embodiment, the repelling field will limit the cathode glow 32 to the anode side of the cathode segment 29 as shown in FIG. 2.

FIG. 3a illustrates a cathode segment 34 to substrate 35 spacing determined by the length of pin 38 on the order of 0.020 inch. Empirically it has been determined that such a spacing provides only a slight, if any, repelling field effect and the cathode glow 36 typically surrounds the cathode segment 34 as shown. When the spacing, as shown in FIG. 3b, is reduced to approximately 0.005 inches, the efiect of the repelling field becomes significant and the cathode glow 36 may extend beyond the horizontal dimensions of the cathode segment 34 but has a minimal extension below the plane defined by the bottom of the cathode segment 34, that is, toward the substrate 35. With spacing of 0.003 inches the effect of the repelling field is very significant as shown in FIG. 30. The horizontal dimensions of the cathode glow 36 may extend somewhat beyond the perimeter of the cathode segment 34 but generally is confined to a smaller horizontal extent because of the repelling field. As the spacing decreases to the point of the cathode 34 being attached to the substrate 35 itself, the repelling field on the substrate 35 proximate to the cathode segment 34 will more strongly confine the cathode glow 36 within the perimeter of the cathode segment 34. In practice, cathode to substrate spacings of less than 0.005 inches provide satisfactory inhibition of cathode glow extending below the cathode segment toward the substrate. As the spacing is reduced to less than 0.003 the clarity of the cathode glow increases, but manufacturing problems may arise in respect to attaching the cathode segments to the pins and in respect to maintaining the cathode segments in an essentially parallel relationship to the substrate. If the cathode to substrate spacing is increased above 0.005 inches, the cathode glow will begin to wrap around the cathode segment and provide added cathode glow which is unnecessary for viewing purposes and which requires more current. The cathode segment to substrate spacing might be extended to perhaps 0.010 inches without incurring the full detrimental effects existing at a spacing of 0.020 inches or more. The purpose and function of moats 38 have been discussed previously in respect to FIG. 1.

The operating pressure and current density have definite effects on the size and intensity of the cathode glow. However, these effects are secondary to the nature of the glow due to the cathode segment to substrate spacing and modify rather than control the nature of the glow. The expected operating temperature range must be considered in determining the initial fill pressure. For any given current density and cathode to substrate spacing, a high pressure will cause the cathode glow to contract from that otherwise possible in respect to the repelling field. A reduction in pressure will cause the cathode glow to billow and increase horizontally and in height above the cathode segment. In the absence of an effective repelling field due to a large cathode segment to substrate spacing, the cathode glow may also extend below the cathode segment. The billowing causes the brightness (intensity per unit area) to decrease and the characters become less sharply defined. The sputter due to the cathode glow is also affected by the pressure in that it increases with lower pressure. Thus, with lower pressure the clarity of the characters and the lifetime of the tube may be decreased. An increase in pressure will cause the cathode glow to contract and it may occur at only a spot on the cathode segment. The exact location is a function of the irregularities of the electrode and generally cannot be determined. In practice, the pressure is selected as high as possible, but constrained so that over the range of operating temperatures, the electrodes will remain covered by the glow despite increasing temperatures and yet prevent the glow from billowing due to a reduction in temperatures.

The degree of cathode glow coverage is also directly related to the current available and the area of the cathode segment. As the cathode to substrate spacing in the apparatus incorporating the instant invention causes the glow to be inhibited from occuring on the substrate side of the cathode segment, the affected area of the cathode segment is limited to the side facing the anode. The current density has a tendency to remain constant and the amount of current necessary for complete cathode glow over the cathode segment is therefore a function of the cathode segment area. Because of the absence of a cathode glow on the underside of the cathode and a minimum of cathode glow in the vertical direction about the perimeter of the cathode segment, essentially all of the glow is used for viewing purposes. The resultant benefit is that of utilizing no more current than that required for presenting a glow generally conforming to the outline of the cathode segment. The efiiciency is thus greater than in devices generating a superfluous portion of cathode glow which is unnecessary and does not contribute to the viewers requirements. Additionally, the current density has a tendency to remain constant and for small currents, the cathode glow may occur at only a spot on the cathode (determined by irregularities of the electrode) and the spot may wander. Thus, other devices having a cathode glow on more than only the viewing side of the cathode segment cannot limit the amount of cathode glow by reduction of current without incurring undesirable flicker or wandering of the cathode glow. As the current is increased the cathode glow in the instant device will extend across the cathode element until inhibited by other considerations such as the repelling field.

The anode to cathode spacing has a secondary rather than a primary effect on the cathode glow and is selected essentially on the following criteria. First, it must be selected so that only cathode glow will occur within the gas discharge tube. Secondly, the anode to cathode spacing is determined by minimizing the voltage required for the expected pressure range in accordance with the Paschen Curve (not shown). A more detailed explanation of the Paschen Curve may be found in Cold Cathode Discharge Tubes, J. R. Acton and J. D. Swift, Academic Press, lnc., 1963.

By limiting the cathode glow to the viewing side only, that is, generally coincident with the perimeter of the upper portion of the cathode and toward the viewing side, several advantages are obtained. First the amount of current required for satisfactory visual display is reduced. Secondly, the reduction in current generates less sputtering. As the sputter products are generated from the cathode on the side thereof remote from the substrate, the sputter products tend to be directed away from the substrate and thus tend not to deposit thereon. Thus, the reduction in sputtering and the directed movement of the sputter products increases the life time of the tube. Thirdly, the efficiency of the instant device is approximately twice that of gas discharge displays which do not limit the extent of cathode glow by utilizing the repelling field.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

We claim:

1. A gas discharge indicator comprising a unitary cathode support substrate having a front surface,

a plurality of electrically conductive pins passing through said substrate and hermetically sealed thereto, one end of each said pins extending above said front surface of said substrate so as to lie in a common plane and said front surface having a moat formed around each pin,

a plurality of cathode segments attached to said pin ends, said cathode segments lying in a common plane spaced from said front surface at a distance to preclude said segments from glowing on the side adjacent to said front surface whereby sputtering from said side is minimized and the width of said segments being greater than the width of said moats whereby said cathode segments hinder deposition of sputter products on the surface of said moats,

anode means disposed adjacent said front surface of said substrate and supported in spaced relation to said substrate and said cathode segments, and

transparent means hermetically sealed to said front surface of said substrate and defining an enclosure for a gas at a predetermined glow discharge pressure.

2. A planar gas discharge indicator comprising a sealed envelope containing an ionizable gas including a substrate and a transparent viewing window,

anode means supported within said envelope,

a plurality of electrically conductive pins passing through and hermetically sealed to said substrate, one end of said pins terminating in a substantially common plane within said envelope generally parallel to and spaced from said substrate at a distance no more than 0.010 inches and the 4 surface of said substrate interior of said envelope having a moat formedtherein about each pin, and

a plurality of substantially rigid cathode segments constructed of flat elongated generally rectangular metallic elements attached to and supported by said pin ends so that said segments lie in a common plane and the surface of each of said segments adjacent the substrate is'raised from said substrate surface no more than 0.010 inches whereby the cathode glow in said indicator is inhibited from extending toward said substrate substantially beyond the plane of said cathode elements, each cathode segment having lateral width in said plane greater than the width of said moats and width greater than the segment thickness normal to said plane whereby said cathode segments hinder the deposition of sputter products within said moats.

3. The indicator as claimed in claim 2 wherein said cathode segments are raised from said substrate less than 0.005 inches, whereby the cathode glow is substantially limited to the cathode segment surface facing said transparent member.

4. A glow discharge type tube for displaying symbols or patterns comprising a closed hermetically sealed envelope having a substantially flat non-conductive back base and a transparent front viewing face,

an ionizable gas enclosed within said envelope,

a plurality of electrically conducting cathode supporting connector pins passing in hermetically sealed relation through the back base into the interior of the sealed envelope and having portions exterior of the envelope for selective energization from a source of electrical excitation,

the interior surface of the back base having a depression formed therein contiguously around each cathode sup- Porting p a plurality of relatively spaced flat cathodes positioned to form a pattern and supported on the ends of said pins interior of the envelope in a common plane spaced from and substantially parallel to the back base, each cathode forming an elemental part of a desired pattern and having a width in said common plane greater than .the width of said depressions whereby the flat surface area of the cathodes effectively shields a portion of the interior surface of said back base to inhibit deposition of cathode sputter products within said depressions, and

anode means disposed within said envelope in spaced relation to said cathodes. 

1. A gas discharge indicator comprising a unitary cathode support substrate having a front surface, a plurality of electrically conductive pins passing through said substrate and hermetically sealed thereto, one end of each said pins extending above said front surface of said substrate so as to lie in a common plane and said front surface having a moat formed around each pin, a plurality of cathode segments attached to said pin ends, said cathode segments lying in a common plane spaced from said front surface at a distance to preclude said segments from glowing on the side adjacent to said front surface whereby sputtering from said side is minimized and the width of said segments being greater than the width of said moats whereby said cathode segments hinder deposition of sputter products on the surface of said moats, anode means disposed adjacent said front surface of said substrate and supported in spaced relation to said substrate and said cathode segments, and transparent means hermetically sealed to said front surface of said substrate and defining an enclosure for a gas at a predetermined glow discharge pressure.
 2. A planar gas discharge indicator comprising a sealed envelope containing an ionizable gas including a substrate and a transparent viewing window, anode means supported within said envelope, a plurality of electrically conductive pins passing through and hermetically sealed to said substrate, one end of said pins terminating in a substantially common plane within said envelope generally parallel to and spaced from said substrate at a distance no more than 0.010 inches and the surface of said substrate interior of said envelope having a moat formed therein about each pin, and a plurality of substantially rigid cathode segments constructed of flat elongated generally rectangular metallic elements attached to and supported by said pin ends so that said segments lie in a common plane and the surface of each of said segments adjacent the substrate is raised from said substrate surface no more than 0.010 inches whereby the cathode glow in said indicator is inhibited from extending toward said substrate substantially beyond the plane of said cathode elements, each cathode segment having lateral width in said plane greater than the width of said moats and width greater than the segment thickness normal to said plane whereby said cathode segments hinder the deposition of sputter products within said moats.
 3. The indicator as claimed in claim 2 wherein said cathode segments are raised from said substrate less than 0.005 inches, whereby the cathode glow is substantially limited to the cathode segment surface facing said transparent member.
 4. A glow discharge type tube for displaying symbols or patterns comprising a closed hermetically sealed envelope having a substantially flat non-conductive back base and a transparent front viewing face, an ionizable gas enclosed within said envelope, a plurality of electrically conducting cathode supporting connector pins passing in hermetically sealed relation through the back base into the interior of the sealed envelope and having portions exterior of the envelope for selective energization from a source of electrical excitation, the interior surface of the back base having a depression formed therein contiguously around each cathode supporting pin, a plurality of relatively spaced flat cathodes positioned to form a pattern and supported on the ends of said pins interior of the envelope in a common plane spaced from and substantially parallel to the back base, each cathode forming an elemental part of a desired pattern and having a width in said common plane greater than the width of said depressions whereby the flat surface area of the cathodes effectively shields a portion of the interior surface of said back base to inhibit deposition of cathode sputter products within said depressions, and anode means disposed within said envelope in spaced relation to said cathodes. 